feat: add GitLab merge request support

[HartBrook]

Summary

• add GitLab.com and trusted self-managed GitLab merge request acquisition, review, and publication
• preserve pinned diff refs through acquisition and recheck them before GitLab bulk publication
• add GitLab-aware launcher, CLI, storage, and host-specific UI behavior
• require glab 1.100+ and GitLab 19.2+ for the API features used

Verification

• npm test (160 tests)
• npm run typecheck
• npm run build
• npm run test:e2e (4 tests)
• npm run test:package
• git diff --cached --check

A colleague who uses GitLab will perform the final authenticated GitLab smoke test.

Risk and privacy

GitLab metadata is read through authenticated glab; source remains local and is fetched into private refs without checkout or code execution. Publishing is explicit and uses the reviewer's GitLab identity. Self-managed hosts must be allowlisted with WINGDIFF_GITLAB_HOSTS. GitLab bulk publication has no revision precondition, so Wingdiff rechecks immediately before publication and documents the remaining final-request race.

Checklist

• The change is focused and documented.
• Tests cover behavior changes, or I explained why tests are unnecessary.
• npm test, npm run typecheck, and npm run build pass.
• I added screenshots for visible UI changes. (No structural UI redesign; GitLab labels are covered by Playwright.)
• I did not commit credentials, private source, or local Wingdiff data.